Northwoodsman: "When a meteorologist predicts a 80% chance of rain (or 40%, 60%, 70%, etc.) in a television broadcast viewing area, how should that be interpreted? I always interpreted it as there is an 80% chance that if I'm in the viewing area that I may get rain where I'm at. However for years one of our local meteorologists for a major network here in Dallas reminds us weekly that a 80% chance of rains means that 80% of the people watching the broadcast WILL get rain. What are your thoughts, or as a trained meteorologist what is your take?"
The '%" chance system in NWS forecasts for precip (temps and wind also) is based on a grid system. In major population areas the grid is smaller, say, 3*3 miles or so, in less populated areas it's likely 5*5 miles or so. What it means is that if forecast is 80% chance of rainfall = 80% of that certain grid is forecast to get at least measurable rainfall. Trace doesn't cut it. Stats are kept. And I've known of NWS Met's being canned (or transferred) based on too many busts.
I have no clue how TV/Radio networks do it, but it seems they try to follow what the NWS does.
What's always been puzzling to me is the following; say you have a winter time forecast that reads like this- "Cloudy with snow likely after 3AM. New snow accumulations of 1-3" possible. Chance of snowfall 80%."
Isn't any snow accumulation mean that it's 100% snowfall??
The NWS has answered me in saying that their forecast is based on probability in the grid system . rather than pure % in the math realm.
Not saying I agree, but I do get their point.
I hope this helps.
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